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The second problem (UPDATE of the child table) is similarly handled by checking the updated foreign key value. If there is no matching primary key value, the UPDATE statement is rejected with an error message. In Figure 11-1, this means that before a salesperson can be reassigned to a different office, that office must already be in the OFFICES table. Again, this restriction makes sense in the sample database. The third problem (DELETE of a parent row) is more complex. For example, suppose you closed the Los Angeles office and wanted to delete the corresponding row from the OFFICES table in Figure 11-1. What should happen to the two child rows in the SALESREPS table that represent the salespeople assigned to the Los Angeles office Depending on the situation, you might want to: Prevent the office from being deleted until the salespeople are reassigned. Automatically delete the two salespeople from the SALESREPS table as well. Set the REP_OFFICE column for the two salespeople to NULL, indicating that their office assignment is unknown. Set the REP_OFFICE column for the two salespeople to some default value, such as the office number for the headquarters office in New York, indicating that the salespeople are automatically reassigned to that office. The fourth problem (UPDATE of the primary key in the parent table) has similar complexity. For example, suppose for some reason you wanted to change the number of the Los Angeles office from 21 to 23. As with the previous example, the question is what should happen to the two child rows in the SALESREPS table that represent salespeople from the Los Angeles office. Again, there are four logical possibilities: Prevent the office number from being changed until the salespeople are reassigned. In this case, you should first add a new row to the OFFICES table with the new office number for Los Angeles, then update the SALESREPS table, and finally delete the old OFFICES row for Los Angeles. Automatically update the office number for the two salespeople in the SALESREPS table, so that their rows are still linked to the Los Angeles row in the OFFICES table, via its new office number. Set the REP_OFFICE column for the two salespeople to NULL, indicating that their office assignment is unknown. Set the REP_OFFICE column for the two salespeople to some default value, such as the office number for the headquarters office in New York, indicating that the salespeople are automatically reassigned to that office. Although some of these alternatives may seem more logical than others in this particular example, it s relatively easy to come up with examples where any one of the four possibilities is the right thing to do, if you want the database to accurately model the real-world situation. The original SQL1 standard provided only the first possibility for the preceding examples it prohibited the modification of a primary key value that was in use and prohibited the deletion of a row containing such a primary key. DB2, however, permitted other options through its concept of delete rules. Starting with SQL2, the SQL standard expanded these delete rules into delete and update rules that cover both deleting of parent rows and updating of primary keys.
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For each parent/child relationship created by a foreign key in a database, the SQL standard allows you to specify an associated delete rule and an associated update rule. The delete rule tells the DBMS what to do when a user tries to delete a row of the parent table. These four delete rules can be specified: RESTRICT delete rule The RESTRICT delete rule prevents you from deleting a row from the parent table if the row has any children. A DELETE statement that attempts to delete such a parent row is rejected with an error message. Deletions from the parent table are thus restricted to rows without any children. Applied to Figure 11-1, this rule can be summarized as You can t delete an office if any salespeople are assigned to it. CASCADE delete rule The CASCADE delete rule tells the DBMS that when a parent row is deleted, all of its child rows should also automatically be deleted from the child table. For Figure 11-1, this rule can be summarized as Deleting an office automatically deletes all the salespeople assigned to that office. SET NULL delete rule The SET NULL delete rule tells the DBMS that when a parent row is deleted, the foreign key values in all of its child rows should automatically be set to NULL. Deletions from the parent table thus cause a set to NULL update on selected columns of the child table. For the tables in Figure 11-1, this rule can be summarized as If an office is deleted, indicate that the current office assignment of its salespeople is unknown. SET DEFAULT delete rule The SET DEFAULT delete rule tells the DBMS that when a parent row is deleted, the foreign key values in all of its child rows should automatically be set to the default value for that particular column. Deletions from the parent table thus cause a set to DEFAULT update on selected columns of the child table. For the tables in Figure 11-1, this rule can be summarized as If an office is deleted, indicate that the current office assignment of its salespeople is the default office specified in the definition of the SALESREPS table. The SQL standard actually calls the previously described RESTRICT rule, NO ACTION. The SQL standard naming is somewhat confusing. It means If you try to delete a parent row that still has children, the DBMS will take no action on the row. The DBMS will, however, generate an error code. Intuitively, the name that DB2 and others use for the rule, restrict, seems a better description of the situation the DBMS will restrict the DELETE operation from taking place and generate an error code. Recent releases of DB2 support both a RESTRICT and a NO ACTION delete rule. The difference between them is the timing of the enforcement of the rule. The RESTRICT rule is enforced before any other constraints; the NO ACTION rule is enforced after other referential constraints. Under almost all circumstances, the two rules operate identically. As you can imagine, support for the delete rules varies among SQL implementations. Table 11-1 shows which rules are supported in current versions of popular DBMS products.
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